1. Field of the Invention
The present invention relates to a power semiconductor switching device, and particularly to a pressurized contact type double gate static induction thyristor.
2. Description of the Related Art
Power semiconductor switching devices are required to permit the flow of as heavy current as several thousand amperes when turning on at power line and to withstand as high voltage as several thousand volts when turning off at power line. In an attempt to meet such strict requirements in power switching application to power line, a variety of semiconductor configurations have been hitherto proposed, and among these proposals a pressurized contact type power semiconductor switching device is the most attractive one. Gate turnoff thyristors, transistors and diodes of the pressurized contact configuration are disclosed, for instance, in Japanese Patent Publn. Nos. 59-29143 and 59-50114.
FIG. 5 shows, in section, a pressurized contact type gate turnoff thyristor. In the drawing, an NPNP lamination body composed of semiconductor layers of different conductivity types is designated at Reference Numeral 100. The NPNP lamination body 100 has a number of cathode projection areas 51 formed an the top surface of the lamination body. An underlying layer of p-type 52 constitutes a gate area and under the p-type layer 52 there is an n-type layer whose bottom is contiguous to a lowermost layer of p-type 53 which constitutes an anode area. Each cathode projection area 51 has a cathode electrode 54 formed thereon. Likewise, the gate area 52 has a gate electrode 55 formed thereon and the anode area 53 has an anode electrode 56 formed thereon. A first thermal expansion stress buffer plate 57 is contacted to the anode electrode 56, and the NPNP lamination body 100 is covered at its peripheral side surface with an insulating resin coating 58. A second thermal expansion stress buffer plate 59 is located on the cathode electrode 54.
The above mentioned structure is contained in a hermatically sealed housing 200 which is composed of an insulating cylinder 60 of ceramic or other insulating material. This insulating cylinder 60 is closed at its top by a cover plate 62 which is secured at its outer periphery at the top end of the cylinder 60. This cover plate 62 supports at its center a cooled cathode electrode 61 in contact to the stress buffer plate 59. Further, the insulating cylinder 60 is closed at its bottom by a flexible cover plate 64 whose outer periphery is fixed to the lower end of the cylinder 60. This cover plate 64 is provided at its center with a cooled anode electrode 63 which is forcedly contacted to the stress buffer plate 57. The cylinder 60 is also provided with a gate electrode connection sleeve 65 which is connected through lead wires 66 to the gate electrode 55. These lead wires 66 are bonded in the conventional method.
The first thermal expansion stress buffer plate 57 is made of a metal whose thermal expansion coefficient is matched to be substantially equal to the thermal expansion coefficient of the semiconductor material of the lamination body 100. Molybdenum or tungsten is used if the semiconductor is silicon. In general, a circular NPNP lamination body is soldered to a first circular, stress buffer plate 57 with aluminum. The circular lamination body 100 has a diameter smaller than the diameter of the circular stress buffer plate 57. As shown, the circumference of the circular lamination body 100 is chamfered to form angles .theta..sub.1 and .theta..sub.2 with respect to the bottom of the body. These chamferings have a great influence on the withstand voltage of the gate turnoff thyristor. Therefore, the circumference of the lamination body is coated with the resin material 58 because a surface discharge would otherwise appear on the lamination body when several thousand voltage is applied across the semiconductor device.
In the pressurized contact configuration as shown, the cooled electrode 61, the second thermal expansion stress buffer 59 and the cathode electrode 54 are slidably contact with each other. Likewise, the first thermal expansion stress buffer plate 57 and the cooled anode electrode 63 are slidably contact with each other. If a stress is generated by the difference between the thermal expansion coefficient of the semiconductor material and that of the cooled electrode when an electric current of several thousand amperes flows in the semiconductor device to raise its temperature, the slidable contact is effective to prevent such stress from applying to the NPNP lamination body 100 of the semiconductor device. In order to reduce the electric resistance at the slidable contact regions of the semiconductor device prior to the flow of a heavy current, a force ranging from several hundred kilograms to several tons is applied to the semiconductor device in the direction as indicated by the arrows in FIG. 5. Then, the first thermal expansion stress buffer plate 57 is effective to prevent the lamination body 100 from being destroyed, and at the same time is effective to protect the circumference of the lamination body 100.
In the pressurized contact configuration as described above, the semiconductor lamination body 100 needs to be fixed to the thermal expansion stress buffer plate by soldering. In the gate turnoff thyristor, only the anode 53 of the lamination structure needs to be fixed to the thermal expansion stress buffer plates, which may be made of single metal plate.
Compared with the conventional gate turnoff structure as described above, a double gate static induction thyristor has a complicated structure. In other words, two different electrodes, i.e. anode and second gate electrodes are formed on one side of the thyristor structure to which a thermal expansion stress buffer plate needs to be fixed. Therefore, the conventional pressurized contact structure cannot be applied as it is to the double gate static induction thyristor. Because of this, the double gate static induction thyristor have not been widely used in practice, although they have been recognized to be more excellent than the conventional gate turn off thyristors.